/*
 * Copyright (C) 2015 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.squareup.gifencoder;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

/**
 * Implements median cut quantization.
 *
 * <p>The algorithm works as follows:
 *
 * <ul>
 *   <li>Begin with one cluster containing all the original colors.</li>
 *   <li>Find the cluster containing the greatest spread along a single color component (red, green
 *   or blue).</li>
 *   <li>Find the median of that color component among colors in the cluster.</li>
 *   <li>Split the cluster into two halves, using that median as a threshold.</li>
 *   <li>Repeat this process until the desired number of clusters is reached.</li>
 * </ul>
 */
public final class MedianCutQuantizer implements ColorQuantizer {
  public static final MedianCutQuantizer INSTANCE = new MedianCutQuantizer();

  private MedianCutQuantizer() {
  }

  @Override public Set<Color> quantize(Multiset<Color> originalColors, int maxColorCount) {
    TreeSet<Cluster> clusters = new TreeSet<>(new ClusterSpreadComparator());
    clusters.add(new Cluster(originalColors));

    while (clusters.size() < maxColorCount) {
      Cluster clusterWithLargestSpread = clusters.pollFirst();
      clusters.addAll(clusterWithLargestSpread.split());
    }

    Set<Color> clusterCentroids = new HashSet<>();
    for (Cluster cluster : clusters) {
      clusterCentroids.add(Color.getCentroid(cluster.colors));
    }
    return clusterCentroids;
  }

  private static final class Cluster {
    final Multiset<Color> colors;
    double largestSpread;
    int componentWithLargestSpread;

    Cluster(Multiset<Color> colors) {
      this.colors = colors;
      this.largestSpread = -1;
      for (int component = 0; component < 3; ++component) {
        double componentSpread = getComponentSpread(component);
        if (componentSpread > largestSpread) {
          largestSpread = componentSpread;
          componentWithLargestSpread = component;
        }
      }
    }

    double getComponentSpread(int component) {
      double min = Double.POSITIVE_INFINITY;
      double max = Double.NEGATIVE_INFINITY;
      for (Color color : colors) {
        min = Math.min(min, color.getComponent(component));
        max = Math.max(max, color.getComponent(component));
      }
      return max - min;
    }

    Collection<Cluster> split() {
      List<Color> orderedColors = new ArrayList<>(colors);
      Collections.sort(orderedColors, new ColorComponentComparator(componentWithLargestSpread));
      int medianIndex = orderedColors.size() / 2;
      return Arrays.asList(
          new Cluster(new HashMultiset<>(orderedColors.subList(0, medianIndex))),
          new Cluster(
              new HashMultiset<>(orderedColors.subList(medianIndex, orderedColors.size()))));
    }
  }

  /**
   * Orders clusters according to their maximum spread, in descending order.
   */
  static final class ClusterSpreadComparator implements Comparator<Cluster> {
    @Override public int compare(Cluster a, Cluster b) {
      double spreadDifference = b.largestSpread - a.largestSpread;
      if (spreadDifference == 0) {
        return ArbitraryComparator.INSTANCE.compare(a, b);
      }
      return (int) Math.signum(spreadDifference);
    }
  }

  /**
   * Orders colors according to the value of one particular component, in ascending order.
   */
  static final class ColorComponentComparator implements Comparator<Color> {
    final int component;

    ColorComponentComparator(int component) {
      this.component = component;
    }

    @Override public int compare(Color a, Color b) {
      double componentDifference = a.getComponent(component) - b.getComponent(component);
      if (componentDifference == 0) {
        return ArbitraryComparator.INSTANCE.compare(a, b);
      }
      return (int) Math.signum(componentDifference);
    }
  }
}
